Python script: Give every scene a unique number
You have added a property called mhfc_uuid1 to the Scene class and set the default value to the pre-calculated value of p_uuid1. Every scene will then get this same value when it is created.

You may want to set the default to zero and then before using it check if it is zero and then set it to a generated value.

class MyOperator(bpy.types.Operator):

    def execute(self, context):
        if context.scene.mhfc_uuid1 == 0:
            context.scene.mhfc_uuid1 = generateuuid()
        do_interesting_stuff()
        return {'FINISHED'}


You could setup existing scenes as your addon is enabled by adding something like this after you add the properties.

for sc in bpy.data.scenes:
    sc.mhfc_uuid1 = generateuuid()